The United States will debate limited options for a military strike on Syria in response to a chemical attack on Aug. 21 that the United States government says killed more than 1,400 people, more than 400 of them children. Following is a brief Q. and A. about why the United States might be headed for a new involvement in the Middle East.

The Obama administration has sought to reduce American military entanglements abroad, withdrawing forces from Iraq two years ago and moving to do the same in Afghanistan next year. So how has it come to pass that we are now moving toward another military entanglement, this time in Syria?

The Syrian conflict affects American foreign policy in a number of ways, but the Obama administration has approached it cautiously, not only because it wants to reduce American military engagement overseas but also because the conflict is complicated and quickly shifting. The rise of sectarianism and the slow collapse of the state in Syria poses a danger to the Middle East as a whole. Millions of refugees have fled to neighboring nations, including Jordan, Lebanon and Turkey, destabilizing them and inflaming sectarian tensions. Violence has also sporadically spilled over into Iraq, Lebanon and Turkey and even Israel, raising the specter of a broader conflict.

On Aug. 21, the Syrian government is believed to have killed hundreds of civilians in a chemical attack on the Sunni-majority suburbs of the capital, Damascus, violating international law and crossing what the Obama administration has called a “red line.” It is believed to be the largest chemical weapons attack since Saddam Hussein of Iraq used a gas attack on the Kurds in 1988, and some observers, including senior Israeli officials, have argued that allowing it to go unpunished sets a dangerous precedent for Syria and its main ally, Iran, suggesting that the use of chemical agents could be tolerated in the future.

How did the conflict in Syria begin, who are the antagonists, and why are they still fighting?

The conflict in Syria grew out of the 2011 Arab Spring protests, when Syrians peacefully demonstrated in towns across the country against Mr. Assad, who succeeded his father, Hafez al-Assad, as president; between the two, the family has held the presidency for four decades. Unlike some other countries facing democratic protests, the Syrian government responded with violence, killing many protesters and radicalizing the movement. Civilians began to take up arms, at first to defend their demonstrations and later to fight security forces in their cities and towns. This nascent armed movement was at first bolstered by army defectors who organized themselves, with Turkish help, under the umbrella of the Free Syrian Army, but over time radical Islamists, including some allied with Al Qaeda, came to play a dominant role, defeating government forces on the battlefield in some towns in the north and east and imposing their rule there.

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American presidents generally want support from three places: Congress, the American people and allies abroad. In the case of a possible military strike in Syria, support may be in short supply.Published OnAug. 31, 2013CreditCreditEvan Vucci/Associated Press

Where does the conflict currently stand?

Today, the Assad government remains the strongest single actor in the conflict, although it has lost a significant amount of territory in the north and east and faces a stalemate against rebels in important areas of the country, including Aleppo and the suburbs of Damascus. Nevertheless, it has a strong arsenal, including chemical weapons, and robust support from its main allies, Russia and Iran. The rebels remain divided among hundreds of small militias and brigades, the most powerful of which are radical Islamist groups. They control much of the country’s north and east, including its borders with Turkey and Iraq, and have begun to enforce Islamic law in some towns. The more secular rebels aligned with the Free Syrian Army are active in towns and suburbs in the south, including the areas of Damascus targeted by the suspected chemical attack, but they are generally weaker than their Islamist counterparts.

Who outside Syria supports the rebel coalition and who supports the government?

Russia and Iran are the Syrian government’s two most important allies, providing financial and military support. Russia additionally provides Syria with important diplomatic cover, including the potential use of its veto on the United Nations Security Council. The largest sources of support for Syria’s rebels have been Turkey and conservative Sunni monarchies in the Persian Gulf, including Saudi Arabia and Qatar. Western countries have been more reluctant to embrace the rebels, providing primarily diplomatic support, in part because of the increasing influence of radical Islamists in the diverse and fragmented coalition of anti-Assad groups.

The conflict has been growing in intensity and scope for more than two years, with the United Nations estimating more than 100,000 dead and millions displaced. Why would the government use chemical weapons now?

There are a number of theories about why the Syrian government might have chosen to use chemical weapons at this point, just days after United Nations weapons inspectors arrived to investigate earlier allegations of chemical weapons use. One theory proposed by a senior Israeli official is that the attack in the Damascus suburbs may have been a miscalculation: Syria may have been using chemical agents on a smaller scale for some time, and used an unintentionally large amount in last week’s attack. “Maybe they were trying to hit one place or to get one effect and they got a much greater effect than they thought,” said the official.

Another theory, argued by Juan Cole, a professor of Middle East Studies at the University of Michigan, is that a siege mentality may have contributed to the Assad government’s decision to use chemical weapons. Faced with intractable Sunni rebels in the Damascus suburbs, the Alawite-led government may have decided to send them a message that the capital would be defended at all costs. “It is the typical behavior of a weak regime facing superior demographic forces (the Alawites are far outnumbered by Sunnis) to deploy unconventional weaponry,” Mr. Cole wrote in a blog post.

What do the United States and its allies hope to achieve through military intervention in the Syrian conflict, and what are the risks?

Pentagon officials have said that President Obama is considering limited military action to “deter and degrade” the Syrian government’s ability to deploy chemical weapons. He is not considering a more ambitious air campaign like the one that helped oust Col. Muammar el-Qaddafi in Libya in 2011, nor is he considering any action that would lead to the deployment of American troops in Syria. Any strikes would target the military units that have deployed chemical weapons, said Pentagon officials, as well as their headquarters and rockets or artillery units that could be used to launch them. Any strikes would not target chemical weapons storage facilities, which could have environmental or humanitarian consequences or open up sensitive sites to looters.